A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fre... more A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and
While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnet... more While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called N\'eel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a
Data storage relies on the handling of two states, called bits. The market of mass storage is cur... more Data storage relies on the handling of two states, called bits. The market of mass storage is currently still dominated by magnetic technology, hard disk drives for the broad public and tapes for massive archiving. In these devices each bit is stored in the form of the direction of magnetization of nanosized magnetic domains, i.e. areas of ferromagnetic materials displaying
Controlled switching of Neel caps in Bloch magnetic domain walls
While magnetic hysteresis usually considers magnetic domains, the switching of the core of mag- n... more While magnetic hysteresis usually considers magnetic domains, the switching of the core of mag- netic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Neel caps. We demonstrated the controlled switching of these caps under magnetic field, occurring via the propagation of a
We report on a new mechanism of nanowire formation: during Au deposition on Si(110) substrates, A... more We report on a new mechanism of nanowire formation: during Au deposition on Si(110) substrates, Au-Si droplets grow, move spontaneously, and fabricate a Si nanowire behind them in the absence of Si external flux. Nanowires are formed by Si dissolved from the substrate at the advancing front of the droplets and transported backward to the crystallization front. The droplet shape is determined by the Si etching anisotropy. The nanowire formation can be tuned by changing experimental parameters like substrate temperature and Au deposition rate.
We demonstrate the kinetically controlled growth of one-dimensional, Co nanomagnets with a high l... more We demonstrate the kinetically controlled growth of one-dimensional, Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, while taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.
Micromagnetic Modeling on self-assembled iron nanostructures
ESAIM: Proceedings, 2008
ABSTRACT Single-crystalline Fe dots self-assembled under ultra high vacuum (UHV) are used as a mo... more ABSTRACT Single-crystalline Fe dots self-assembled under ultra high vacuum (UHV) are used as a model system to discuss micromagnetic properties of sub-micron size magnetic dots. Landau and diamond states were identified by magnetic force microscopy (MFM) and reproduced by simulations. To understand mechanisms involved in the magnetization reversal, the in-plane angular variations of nucleation and annihilation fields of a multi-domain magnetic single dot with a micro-SQUID were studied. Des îlots de fer auto-assemblés obtenus sous ultravide (UHV) sont utilisés comme système modèle pour étudier les propriétés micromagnétiques de plots sub-microniques. Les états Landau et losange ont été identifiés et reproduits par simulations. Pour comprendre les mécanismes de renversement de l'aimantation, les variations angulaires des champs de nucléation et d'annihilation d'un plot unique multi-domaines ont été étudiées par microsquid.
is performed in a kinetically-limited regime to yield a nanotemplate in the form of a uniaxial ar... more is performed in a kinetically-limited regime to yield a nanotemplate in the form of a uniaxial array of hills and grooves aligned along the [001] direction. The topography and organization of the grooves were studied with RHEED and STM. The nanofacets, of type {210}, are tilted ∼ 18 • away from (110). The lateral period could be varied from 4 to 12 nm by tuning the deposition temperature. Magnetic nanowires were formed in the grooves by deposition of Fe at 150 • C on such templates. Fe/W wires display an easy axis along [001] and a mean blocking temperature T B ≈ 100 K.
While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnet... more While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Néel caps. We demonstrated the controlled switching of these caps under magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.
Dimensionality crossover is a classical topic in physics. Surprisingly, it has not been searched ... more Dimensionality crossover is a classical topic in physics. Surprisingly, it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was confirmed experimentally based on micron-sized flux-closure dots.
We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, usi... more We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, using scanning tunneling microscopy and grazing incidence x-ray diffraction. We provide compelling evidence of an Ag(110) surface reconstruction associated with the release of Ag atoms induced by the growth of Si nanoribbons. Our results are in agreement with a missing row reconstruction of the Ag layer underneath the nanoribbons. This challenges the current understanding of the Si growth on nonreconstructed Ag(110), interpreted within the framework of silicene models.
The dewetting properties of Ge/SiO2 have been studied by low-energy electron microscopy and grazi... more The dewetting properties of Ge/SiO2 have been studied by low-energy electron microscopy and grazing incidence small-angle x-ray scattering in two temperature ranges characterized by the presence or the absence of {15 3 23} facets on the dewetting fronts. Thanks to a comparison with the Si/SiO2 system, we show that the {15 3 23} facets: (i) play a role in the stabilization properties of Ge dewetting fronts, (ii) lead to a rotation of 45◦ of the Ge fingers with respect to the Si fingers, and (iii) increase the Ge fingers’ stability delaying the formation of a three-dimensional Ge islands with respect to Si for the benefit of the formation of Ge nanowires. Studying the dewetting kinetics enables estimating the activation energy for Ge dewetting to 2.7 ± 0.2 eV. The weak energetics differences between Si and Ge systems are sufficient to change the dewetting morphologies from a squared-void opening for Si/SiO2 to multibranch dendrites for Ge with specific consequences on the relative dewetting velocities of the Si/SiO2 and Ge/SiO2 systems.
Using low-energy electronmicroscopy (LEEM), we have measured, in real time, the dewetting of sing... more Using low-energy electronmicroscopy (LEEM), we have measured, in real time, the dewetting of single-crystal Si(001) thin films on amorphous silicon dioxide substrates, which transforms the two-dimensional (2D) thin film into three-dimensional (3D) compact Si nanocrystals. The dewetting scenario has been reported by Bussmann et al. [New J. Phys. 13, 043017 (2011)]. Analytic 2D and 3D models based on simple approximate geometries of the dewetting front have been developed to analyze LEEM measurements. They enable us to estimate the driving force for dewetting Es ∼ 14 eV/nm2. Starting from a Si-film thickness dependent effective dewetting activation barrier, a single Si(001) surface self-diffusion energy of Ea = 2.0 ± 0.2 eV is derived. First nanoisland-formation dynamics measurements are discussed. Finally, grazing incidence small-angle x-ray scattering (GISAXS) is used to characterize the structure and the morphology of the Si nanocrystals created by the dewetting process.
Using low-energy electron microscopy movies, we have measured the dewetting dynamics of single-cr... more Using low-energy electron microscopy movies, we have measured the dewetting dynamics of single-crystal Si(001) thin films on SiO 2 substrates. During annealing (T > 700 • C), voids open in the Si, exposing the oxide. The voids grow, evolving Si fingers that subsequently break apart into selforganized three-dimensional (3D) Si nanocrystals. A kinetic Monte Carlo model incorporating surface and interfacial free energies reproduces all the salient features of the morphological evolution. The dewetting dynamics is described using an analytic surface-diffusion-based model. We demonstrate quantitatively that Si dewetting from SiO 2 is mediated by surface-diffusion driven by surface free-energy minimization. S Online supplementary data available from stacks.iop.org/NJP/13/043017/ mmedia Solid thin films play a variety of vital technological roles, e.g. as catalytic membranes, and are key building-blocks of microelectronics. Numerous thin-film systems are known to be susceptible to dewetting instabilities, wherein the two-dimensional (2D) film spontaneously agglomerates into compact 3D nanostructures-typically during thermal annealing-which may activate mass-transport processes in or on the film [1]- . From one viewpoint, dewetting is a problem that e.g. complicates the fabrication of thin-film-based devices by imposing constraints on the thermal budget during processing [1]. However, thin-film dewetting 3 Present address:
We report on a new approach, entirely based on an electron-beam lithography technique, to contact... more We report on a new approach, entirely based on an electron-beam lithography technique, to contact electrically, in a four-probe scheme, single nanostructures obtained by self-assembly. In our procedure, nanostructures of interest are located and contacted in the same fabrication step. This technique has been developed to study the field-induced reversal of an internal component of an asymmetric Bloch domain wall observed in elongated structures such as Fe(110) dots. We have focused on the control, using an external magnetic field, of the magnetization orientation within Néel caps that terminate the domain wall at both interfaces. Preliminary magneto-transport measurements are discussed demonstrating that single Fe(110) dots have been contacted.
Influence of Polymer-Blend Morphology on Charge Transport and Photocurrent Generation in Donor−Acceptor Polymer Blends
Nano Letters, 2006
Monte Carlo algorithms are used to simulate the morphologies adopted by polymer chains in a polym... more Monte Carlo algorithms are used to simulate the morphologies adopted by polymer chains in a polymer-blend film in the limits where the chains are mutually attractive (homophilic regime) and mutually repulsive (heterophilic regime) and then to simulate the drift transport of charges through the polymer chains. In the homophilic regime, chains aggregate into tangled domains resulting in a relatively high percolation threshold, a high density of configurational trap states, and slow, dispersive charge transport. In the heterophilic regime at the same polymer volume fraction, chains self-organize into a lacework pattern resulting in a low percolation threshold and efficient, trap-free charge transport. For homophilic morphologies interchain hopping is rate-limiting and mobility is insensitive to chain length, whereas for heterophilic morphologies intrachain transport is important and mobility increases with increasing chain length. The morphologies are used in simulations of photocurrent quantum efficiency for donor-acceptor blend photodiodes, which show that the effects of morphology on charge pair generation and recombination compete with the effect on transport, such that the optimum blend composition is sensitive to both morphology and recombination rate. We conclude that it is essential to consider the connectivity of and morphology adopted by polymer chains in the optimization of materials for organic solar cells.
We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview... more We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski-Krastanov growth mode, recognized for a long time in these systems, is in fact characterized by a bimodal distribution of islands for growth temperature in the range ∼ 250 − 700 • C. We observe firstly compact islands whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat islands that display a preferred height, i.e. independant from nominal thickness and deposition procedure (1.4 nm for Mo, and 5.5 nm for W on the average). We used this effect to fabricate self-organized arrays of nanometers-thick stripes by step decoration. Self-assembled nano-ties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, i.e. ∼ 800 • C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, second that of the continuouslyvarying lattice parameter of the growth surface.
We report a detailed magnetic study of a new type of self-organized nanowires disclosed briefly p... more We report a detailed magnetic study of a new type of self-organized nanowires disclosed briefly previously [B. Borca et al., Appl. Phys. Lett. 90, 142507 (2007)]. The templates, prepared on sapphire wafers in a kinetically-limited regime, consist of uniaxially-grooved W(110) surfaces, with a lateral period here tuned to 15 nm. Fe deposition leads to the formation of (110) 7 nm-wide wires located at the bottom of the grooves. The effect of capping layers (Mo, Pd, Au, Al) and underlayers (Mo, W) on the magnetic anisotropy of the wires was studied. Significant discrepancies with figures known for thin flat films are evidenced and discussed in terms of step anisotropy and strain-dependent surface anisotropy. Demagnetizing coefficients of cylinders with a triangular isosceles cross-section have also been calculated, to estimate the contribution of dipolar anisotropy. Finally, the dependence of magnetic anisotropy with the interface element was used to tune the blocking temperature of the wires, here from 50 K to 200 K.
X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is ... more X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on Néel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of Néel-cap magnetization give clear evidence for the magnetization reversal of the Néel caps around 120 mT, with a ±20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots.
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Papers by Fabien Cheynis
incidence small-angle x-ray scattering in two temperature ranges characterized by the presence or the absence
of {15 3 23} facets on the dewetting fronts. Thanks to a comparison with the Si/SiO2 system, we show that
the {15 3 23} facets: (i) play a role in the stabilization properties of Ge dewetting fronts, (ii) lead to a rotation
of 45◦ of the Ge fingers with respect to the Si fingers, and (iii) increase the Ge fingers’ stability delaying the
formation of a three-dimensional Ge islands with respect to Si for the benefit of the formation of Ge nanowires.
Studying the dewetting kinetics enables estimating the activation energy for Ge dewetting to 2.7 ± 0.2 eV. The
weak energetics differences between Si and Ge systems are sufficient to change the dewetting morphologies from
a squared-void opening for Si/SiO2 to multibranch dendrites for Ge with specific consequences on the relative
dewetting velocities of the Si/SiO2 and Ge/SiO2 systems.
Si(001) thin films on amorphous silicon dioxide substrates, which transforms the two-dimensional (2D) thin film
into three-dimensional (3D) compact Si nanocrystals. The dewetting scenario has been reported by Bussmann
et al. [New J. Phys. 13, 043017 (2011)]. Analytic 2D and 3D models based on simple approximate geometries of
the dewetting front have been developed to analyze LEEM measurements. They enable us to estimate the driving
force for dewetting Es ∼ 14 eV/nm2. Starting from a Si-film thickness dependent effective dewetting activation
barrier, a single Si(001) surface self-diffusion energy of Ea = 2.0 ± 0.2 eV is derived. First nanoisland-formation
dynamics measurements are discussed. Finally, grazing incidence small-angle x-ray scattering (GISAXS) is used
to characterize the structure and the morphology of the Si nanocrystals created by the dewetting process.